The present apparatus relates to automotive exhaust systems, and more particularly relates to catalytic converters and particulate traps that are vacuum insulated and that have phase change materials to assist in retention of heat and further that are adapted to treat emissions, such as are found in gasoline engine exhaust and diesel engine exhaust.
Most vehicle exhaust systems and particularly exhaust systems of vehicles powered by internal combustion engines are equipped with engine exhaust treatment systems for reducing noxious emissions in exhaust gases. Engine exhaust treatment systems consist primarily of a catalytic converter which converts toxic exhaust emissions to non-toxic gases. The oxidative and reductive reactions that convert the emissions occur on the hot catalytically active surface of the converter. A problem exists in that a large part of tailpipe hydrocarbon emissions occur during the initial cold start phase when the catalytic converter is least effective. Until the converter is heated to a sufficiently high temperature, these reactions do not occur efficiently and exhaust gases pass through the system untreated. EPA estimates indicate that as much as 80% of all automobile commuter exhaust emissions occur during the so-called “cold start” period when the catalytic converter is heating up to operational temperature. Where there are oxides of nitrogen that must be reduced, such as in exhaust from diesel engines, there is an upper and lower operating range for optimal operation of the catalytic materials.
A vacuum-insulated catalytic converter with included thermal energy storage improves the efficiency of engine exhaust emissions treatment by remaining hot long after the engine is shut off. If the engine is not shut off for too long, the still hot and catalytically active converter is immediately effective the next time the engine is used and avoids the “cold start” emission of untreated exhaust gases. However, passive thermal energy storage systems have problems. In passive thermal energy storage systems, the thermal energy storage material is well connected thermally to the catalytic converter so that heat will readily flow from the thermal energy storage material to the catalytic converter when it requires heat. But when the converter has sat for a long time and the thermal energy storage material has cooled, this close coupling will draw heat from the hot exhaust gas stream and from the catalyst until the thermal energy storage material itself is heated to a high temperature. This will require some additional time during which untreated exhaust gases will be emitted. Thus, while the above-discussed design is effective in reducing “cold start” emissions when it is hot, it actually exacerbates the “cold start” problem whenever the converter has been allowed to cool. Notably, no matter how effectively the converter stores heat, there will be times when it has cooled and will suffer from some degree of “cold start” emissions.
At least one prior design converter utilized a salt Phase Change Material (PCM) for thermal energy storage. There are some fundamental problems in using a salt PCM. One problem is that the thermal conductivity of a salt is relatively low, resulting in extended vehicle driving times to fully melt the PCM and realize the full heat storage of the PCM. Another problem is the large thermal expansion of most salts from ambient temperature to elevated temperatures typical of an auto exhaust. This expansion can be as high as 30% which results in inefficient packaging since allowance must be made in the PCM container for this expansion. This is undesirable since the area under a vehicle is very confined and at a premium, particularly as vehicles are built to ride closer and closer to the road surface.
It is also known to actively manage a vacuum environment in a catalytic converter. For example, see Benson U.S. Pat. No. 5,477,676. However, the electrically heated and controlled hydrogen source disclosed in Benson U.S. Pat. No. 5,477,676 to prevent the converter from overheating requires complex and expensive wiring to the catalytic converter. This wiring and control system also adds to the vulnerability of the exhaust system to failure and increases the risk of a warranty liability or expensive model recall action by the manufacturer. This can be a serious problem, particularly given the severe environments that exhaust systems are subjected to.
Thermally-activated exhaust treatment devices also include particulate traps for capturing and treating particulate emissions, such as carbon particles and soot from diesel engines. Particulate traps work best at elevated temperature. Particulate traps are least effective at cold starts, which is when the problem of carbon particulate emissions and creation of soot is the greatest in diesel engines. Accordingly there are significant advantages to be achieved in particulate traps by vacuum insulating them to conserve and hold their temperatures longer upon engine shut off.
Accordingly, an exhaust treatment device is desired solving the aforementioned problems and offering the aforementioned advantages.